The invention relates to a method for ensuring the safety in use of a machine tool having a first tool part carrying out working movements in the direction of a second tool part so that, in the course of these working movements, an open gap between the first tool part, on the one hand, and the second tool part or a workpiece to be machined, on the other hand, is gradually closed. In this process, an optoelectronic sensor monitors a three-dimensional protected field between the tool parts in that at least one light ray is transmitted along the open gap between the tool parts by means of a transmitter device and is detected by means of a receiver device. When the sensor detects an interruption of the light ray, a switch-off process is triggered to stop the first tool part in order to prevent a possible injury to an operator of the machine tool.
The invention furthermore relates to a correspondingly configured opto-electronic sensor.
Said machine tool is typically a stamping press for the bending, folding, cutting or stamping of workpieces such as sheet metal parts. Said first tool part is formed in this case by a top tool which has a longitudinal bending line or cutting edge. This top tool is moved, during its working movement, vertically downwardly toward a bottom tool which serves as a second tool part and which contacts or lies on the workpiece.
For each machining process, the previously machined workpiece must first be removed and the new workpiece to be machined must be introduced into the open gap between the tool parts. This is done manually by an operator. To avoid an injury to the operator, in particular to his hands, during the closing of the open gap, the optoelectronic sensor monitors a protected field which typically extends beneath the moved top tool in the case of a stamping press. This protected field is three-dimensional, i.e. at least its outline extends along a two-dimensional cross-section such that the sensor can react fast and reliably to intrusions from different directions.
Such an ensuring of the safety in use of machine tools has proven its value to reduce the explained risk of injury. On the other hand, there is naturally the endeavor that the working movements of the first tool part can be carried out as fast as possible so that the machine tool can work at a high work cycle. However, an increased work cycle conflicts to a certain extent with the monitoring of a large protected field since, with a large protected field, adjustments of the workpiece by the operator which are not dangerous per se can already result in an unwanted switch-off process.
If the extent of the protected field monitored by the sensor is in turn reduced with respect to the direction of the working movement of the first tool part due to these considerations, the further problem exists that possible vibrations of the machine tool can result in the transmitted light ray no longer acting on the receiver device as planned or in its being interrupted by a tool part and that unwanted switch-off processes can there-fore likewise occur.
WO 00/67932, for example, describes a press safety system having an optoelectronic sensor whose transmitter device transmits a laser beam extending along a horizontal plane. A cylindrical lens is provided at the receiver side. It is adapted to simplify an adjustment of the planar laser beam and to make the sensor insensitive with respect to vertical vibrations. However, this safety system has the disadvantage that the risk of incorrect switch-offs due to vertical vibrations is only eliminated to a limited extent and also that only a comparatively small protected zone is available in the vertical direction.